Image display device

Abstract

To provide an image display device which is highly reliable and is bendable.SOLUTION: An image display device (101) includes a polarizer (31) and a cover window (71) in that order on a visually recognition side of an image display panel (51). The image display device also includes a touch panel (41) within a distance 500 μm from a touch surface. A first adhesive sheet (11) is provided on a surface on the visually recognized side of the polarizer, and a second adhesive sheet (12) is provided on a surface on the image display panel side of the polarizer. The first adhesive sheet and the second adhesive sheet have a dielectric constant of not larger than 4.5 when the temperature is 25°C and the frequency is 10 kHz.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a foldable image display device that includes a touch panel. [Background technology]

[0002] Flat panel displays such as liquid crystal displays and organic electroluminescence (EL) displays are used as image display devices for mobile phones, smartphones, tablet terminals, car navigation devices, PC monitors, televisions, etc. In recent years, organic EL panels using foldable substrates (flexible substrates) such as resin films have been put to practical use, and foldable flexible displays have been proposed.

[0003] In a flexible display, in addition to the display panel such as an organic EL panel being bendable, the components such as the housing and touch panel are also bendable, and these components are bonded together via an adhesive sheet (for example, Patent Document 1). In a bendable flexible display (foldable display), the transparent plate (cover window) placed on the viewing side surface must also be bendable, and thin materials such as resin film and thin glass are used.

[0004] In a foldable display, bending is performed repeatedly at the same place. At the bending point, compressive stress is applied to the inside and tensile stress is applied to the outside, which causes distortion at the bending point and its surroundings, raising concerns about the destruction of the device. For this reason, it has been proposed to soften the adhesive sheet that bonds the components together to relieve the stress distortion (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-2764 A [Patent Document 2] JP 2018-45213 A Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, there has been a demand for thinner foldable display devices. In particular, there is a demand for highly reliable foldable display devices that have little or no malfunction of the touch panel sensor even when the distance from the device surface to the touch panel sensor is small, and that suppress or prevent failures. The present invention aims to provide an image display device that is foldable and has high reliability. [Means for solving the problem]

[0007] One embodiment of the present invention is a foldable image display device having a touch panel. The image display device has a polarizing plate and a cover window in this order on the viewing side of an image display panel, and has a touch panel within a distance of 500 μm from the touch surface. The touch panel is disposed, for example, between the image display panel and the polarizing plate. It may also be disposed inside the image display panel.

[0008] A first adhesive sheet is provided on the viewing side surface of the polarizing plate, and a second adhesive sheet is provided on the image display panel side surface of the polarizing plate. Both the first adhesive sheet and the second adhesive sheet have a relative dielectric constant of 4.5 or less at a temperature of 25° C. and 10 kHz.

[0009] The first adhesive sheet and the second adhesive sheet preferably have a ratio of the dielectric constant at 1 kHz to the dielectric constant at a frequency of 1 MHz of 1.50 or less at a temperature of 25° C. For both the first adhesive sheet and the second adhesive sheet, the maximum value of the dielectric constant at a frequency of 10 kHz in the temperature range of -40° C. to 80° C. is preferably 1.4 times or less the minimum value. For both the first adhesive sheet and the second adhesive sheet, the ratio of the maximum value to the minimum value of the dielectric constant at a frequency of 1 kHz in the temperature range of -40° C. to 80° C. is preferably 0.8 to 1.2 times the ratio of the maximum value to the minimum value of the dielectric constant at a frequency of 1 MHz in the temperature range of -40° C. to 80° C.

[0010] The cover window may have a thickness of 100 μm or less.

[0011] The thickness of the first adhesive sheet may be greater than the thickness of the second adhesive sheet, and the thickness of the first adhesive sheet may be 100 μm or less.

[0012] Storage modulus G' of first adhesive sheet at 25°C and 1 Hz 25 The pressure sensitive adhesive sheet may have a glass transition temperature of -20°C or lower.

[0013] The first adhesive sheet may be composed of an acrylic adhesive containing an acrylic base polymer. The acrylic base polymer may be a polymer having a ratio of (meth)acrylic acid C to 100 parts by weight of the total of the monomer components. 10-20 The acrylic base polymer may contain 5 to 55 parts by weight of a chain alkyl ester. 10-20 The chain alkyl ester may include lauryl acrylate.

[0014] The acrylic base polymer may contain 2 to 15 parts by weight of one or more polar group-containing monomers selected from the group consisting of hydroxy group-containing monomers, carboxy group-containing monomers, and nitrogen-containing monomers, based on 100 parts by weight of the total of the monomer components. The acrylic base polymer may contain 10 parts by weight or less of the hydroxy group-containing monomer, based on 100 parts by weight of the total of the monomer components.

[0015] The acrylic base polymer may have a crosslinked structure. The crosslinked structure may be introduced by a multifunctional (meth)acrylate.

[0016] The acrylic pressure-sensitive adhesive may further contain an acrylic oligomer having a glass transition temperature of not less than 60° C. The content of the acrylic oligomer relative to 100 parts by weight of the acrylic base polymer may be 0.1 to 5 parts by weight. Effect of the Invention

[0017] The image display device of the present invention reduces malfunctions of the touch panel and can exhibit high reliability. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view showing an example of the configuration of an image display device. [Diagram 2] FIG. 1 is a cross-sectional view showing an example of the configuration of an image display device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] 1 and 2 are cross-sectional views of a flexible display according to an embodiment.

[0020] 1, an organic EL panel 51, a touch panel 41, and a circular polarizer 31 are disposed between a housing 75 and a cover window 71. The organic EL panel 51 and the bottom surface of the housing 75 are bonded together via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded together via an adhesive sheet 13, the touch panel 41 and the circular polarizer 31 are bonded together via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via an adhesive sheet 11. In this way, in a flexible display, a plurality of members are bonded together via the adhesive sheets, thereby forming an integrated laminate.

[0021] The cover window 71 is disposed on the viewing side surface of the flexible display and constitutes a touch surface. The touch panel 41 is a capacitive touch panel. Since the flexible display uses a foldable cover window 71, the thickness of the cover window is small, and accordingly, the distance D from the touch surface (the surface of the cover window 71) to the touch panel 41 is also small.

[0022] 2 includes an organic EL panel 54 integrated with a touch panel, and a circular polarizer 31 is attached onto the organic EL panel 54 via an adhesive sheet 12. The rest of the configuration is the same as in Fig. 1, and the distance from the touch surface to the viewing side surface of the organic EL panel 54 corresponds to the distance D from the touch surface to the touch panel.

[0023] In an image display device using a rigid glass plate as a cover window, the distance from the touch surface to the touch panel is generally 700 μm or more, whereas in a flexible display, the distance D from the touch surface to the touch panel 41 (the distance from the touch surface to the image display panel when the touch panel is an in-cell type) is 500 μm or less. The distance D from the touch surface to the touch panel may be 400 μm or less, 350 μm or less, or 300 μm or less.

[0024] 1 and 2, the image display device includes a polarizing plate 31 and a cover window 71 on the viewing side of image display panels 51, 54. A first adhesive sheet 11 is provided on the viewing side surface of the polarizing plate 31, and a second adhesive sheet 12 is provided on the surface of the polarizing plate 31 facing the image display panels 51, 54.

[0025] [Dielectric constant of adhesive sheet] The adhesive sheets 11 and 12 arranged on the upper and lower surfaces of the polarizing plate 31 have a relative dielectric constant of 4.5 or less at a temperature of 25° C. and a frequency of 10 kHz. In the following, unless otherwise specified, the dielectric constant is a measured value at a temperature of 25° C. The relative dielectric constant of the adhesive sheet at a frequency of 10 kHz may be 4.0 or less, 3.8 or less, or 3.5 or less. By making the relative dielectric constant at a frequency of 10 kHz 4.5 or less, the distance from the touch surface to the touch panel can be reduced, making it possible to design advantageously for flexibility. Since an adhesive sheet with a low dielectric constant has a small electrostatic capacitance value, it is possible to design a sensor with good sensitivity. This enables an input method with a small contact area, such as pen input.

[0026] The dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 1 kHz is preferably 5.0 or less, more preferably 4.8 or less, and may be 4.5 or less, 4.0 or less, or 3.8 or less. The dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 100 kHz is preferably 4.0 or less, and may be 3.8 or less, or 3.5 or less. The dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 1 MHz is preferably 3.5 or less, and may be 3.3 or less, or 3.2 or less.

[0027] The dielectric constant changes depending on the polarizability of the material, and the dielectric constant of the adhesive sheet can be controlled by selecting the adhesive material such as urethane, acrylic, rubber, and silica. Also, since the relative dielectric constant of air is 1, the dielectric constant of the adhesive sheet can be reduced by adding hollow beads to the adhesive. In acrylic adhesives, monomers with long alkyl chains have low polarizability and can be made low dielectric. When a highly polar monomer is used, the polarizability is high and the dielectric constant becomes high. One method of reducing the polarizability is to cause molecular entanglement. Increasing the molecular weight or the degree of crosslinking makes it easier for molecular entanglement to occur, which reduces the polarizability and allows for low dielectric constant. Since the dielectric constant tends to increase as the moisture content increases, the dielectric constant can be reduced by using a material that does not easily retain moisture.

[0028] According to the Clausius-Mossotti formula, the smaller the polarizability of the electric dipole and the smaller the number of electric dipoles per unit volume, the smaller the relative dielectric constant. In order to reduce the relative dielectric constant of the adhesive sheet, the dipole moment of the base polymer constituting the adhesive may be reduced and the molar volume may be increased. For example, the larger the volume of the side chain of the base polymer, the larger the molar volume tends to be. In addition, by selecting a monomer with a small polarity as the monomer component constituting the base polymer, the electronic dipole of the molecule is reduced.

[0029] The adhesive sheet preferably has a small frequency dependency of the dielectric constant at a temperature of 25°C. Specifically, the ratio of the dielectric constant at a frequency of 1 kHz to the dielectric constant at a frequency of 1 MHz (1 kHz / 1 MHz) of the adhesive sheet is preferably 1.5 or less. The small dielectric constant over a wide frequency range and the small frequency dependency of the dielectric constant ensure operational reliability at various operating frequencies. As described above, the frequency dependency of the dielectric constant can be reduced by adjusting the dielectric constant of the adhesive.

[0030] It is preferable that the adhesive sheet has a small temperature dependency of the dielectric constant. Specifically, it is preferable that the ratio of the minimum value to the maximum value of the dielectric constant (maximum value / minimum value) in the temperature range of -40°C to -80°C is close to 1. The ratio X of the maximum value to the minimum value of the dielectric constant at a frequency of 10 kHz 10kHz is preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and may be 1.1 or less. 1kHz , the ratio of the maximum and minimum values ​​of the relative dielectric constant at a frequency of 100 kHz, X 100kHz , and the ratio of the maximum and minimum values ​​of the relative dielectric constant at a frequency of 1 MHz, X 1MHz are each preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and may be 1.1 or less.

[0031] The ratio X of the maximum and minimum values ​​of the dielectric constant in the temperature range of -40°C to -80°C is an index of the temperature dependency of the dielectric constant, and the closer X is to 1, the smaller the temperature dependency of the dielectric constant. As described above, in addition to the small temperature dependency of the dielectric constant at a frequency of 10 kHz, it is preferable that the temperature dependency of the dielectric constant is small over the entire frequency range of 1 kHz to 1 MHz. When the temperature dependency of the dielectric constant is small, the frequency dependency also tends to be small. Also, the higher the frequency, the temperature at which the dielectric constant is maximized tends to shift to the high temperature side.

[0032] X above 1MHz and X 1kHz Ratio to X 1kHz / X 1MHzis preferably 0.8 to 1.2, more preferably 0.85 to 1.15, and further preferably 0.9 to 1.1, and may be 0.95 to 1.05. 1kHz / X 1MHz The closer to 1, the smaller the change in the relative dielectric constant is over a wide temperature range and frequency range, so that operational reliability can be ensured over a wide temperature range and operating frequency range.

[0033] [First Adhesive Sheet] The first adhesive sheet 11 arranged on the viewing side of the polarizing plate 31 preferably has a gap length of 2 mm or less in a bending retention test described below (i.e., the gap length between the adhesive sheet and the adherend after 240 hours of bending retention: hereinafter sometimes simply referred to as "gap distance"). If the gap distance is 2 mm or less, even in a foldable image display device having a touch panel within a distance of 500 μm from the touch surface, the adhesive sheet 11 can absorb stress caused by bending even when the image display device is folded, and the reliability of the image display device can be improved. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is little or no malfunction of the touch panel sensor, and failures and the like are suppressed or prevented, resulting in high reliability.

[0034] The gap distance is preferably 1.5 mm or less, more preferably 1.0 mm or less, and may be 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. The lower limit of the gap distance is not particularly limited, and may be 0.

[0035] Peeling (gaps) in the bending retention test is likely to occur from the end of the bending axis of the test piece (end in the short side direction). When gaps occur from both ends or when there are gaps in multiple places, the length of the gap with the longest length in the short side direction is taken as the gap distance. When there are gaps in multiple places, the longest gap may be 2 mm or less. Preferably, the total length of each gap is 2 mm or less, and the total length of the gaps may be 1.5 mm or less, 1.0 mm or less, 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less, or may be 0.

[0036] When bending, the adherend expands and contracts, and the adhesive cannot follow the deformation of the adherend, causing peeling from the adherend. For example, the adhesive's ability to follow the adherend can be adjusted by controlling the storage modulus G' of the adhesive. The smaller the storage modulus of the adhesive, the better the ability to follow the deformation of the adherend. When the bent state is maintained, peeling may occur due to the concentration of stress accompanying the deformation of the bent portion. In order to increase the stress relaxation of the adhesive, the loss tangent tanδ may be increased. In addition, in order to suppress peeling at the interface between the adherend and the adhesive, it is also necessary to design the adhesive to have a high adhesive strength at the bending temperature. Furthermore, in a high humidity environment, the retention of moisture at the interface between the adhesive and the adherend can cause a decrease in adhesive strength, so it is preferable to use a material that does not easily retain moisture to suppress the retention of moisture.

[0037] The adhesive strength of the first adhesive sheet 11 to the polyimide film is preferably 2.7 N / 10 mm or more, more preferably 2.8 N / 10 mm or more, and may be 3.0 N / 10 mm or more. The adhesive strength is determined by a peel test using a polyimide film as an adherend at a tensile speed of 60 mm / min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is a value measured at 25° C. When the adhesive sheet 11 has an adhesive strength in the above range, peeling of the adherend, such as the polarizing plate 31 or the cover window 71, can be prevented when the sheet is repeatedly bent.

[0038] The adhesive sheet 11 has a storage modulus G' at 25°C. 25 It is preferable that G' is 70 kPa or less. 25 By making the G' equal to or less than 70 kPa, distortion when the device is bent tends to be alleviated, and damage to the device components when the device is repeatedly bent can be suppressed. 25 In this range, the adhesive sheet can have both the adhesive holding power and the strain relaxation property, and therefore the gap distance can be made small. 25 The pressure is preferably from 10 to 60 kPa, more preferably from 13 to 50 kPa, and even more preferably from 15 to 40 kPa.

[0039] The adhesive sheet 11 has a storage modulus G' at 100°C. 100 The G' of the pressure-sensitive adhesive sheet is preferably 2 to 50 kPa, more preferably 3 to 40 kPa, and even more preferably 5 to 25 kPa. 100 When the thickness is within the above range, both the adhesive holding strength and the stress relaxation property can be achieved even in a high temperature environment, so that the gap distance can be made small.

[0040] Loss tangent tanδ of adhesive sheet 11 at 25°C 25 The loss tangent tan δ at 100° C. is preferably 0.2 to 0.45. 100 is preferably 0.2 to 0.4. 25 and tan δ 100 The difference between tan δ and tan δ is preferably −0.07 to 0.07. 25 may be 0.25 to 0.42. 100 may be 0.25 to 0.38. 25 and tan δ 100 The difference between the tan δ of the pressure-sensitive adhesive sheet and the tan δ of the pressure-sensitive adhesive sheet may be within ±0.06 or ±0.05. 25 , tan δ 100 , and tan δ 25 and tan δ 100 By keeping the difference between the tan δ and the tan δ within the above range, the gap distance can be reduced. The tan δ of the pressure-sensitive adhesive sheet can be adjusted by optimizing the material monomer and the degree of crosslinking. For example, the material monomer can be selected so that the glass transition temperature and molecular weight of the base polymer are in an appropriate range.

[0041] The storage modulus G' and loss tangent tanδ of the adhesive sheet 11 are determined by viscoelasticity measurement at a frequency of 1 Hz. Tanδ is the ratio G" / G' of the storage modulus G' to the loss modulus G". The storage modulus G' corresponds to the portion stored as elastic energy when the material deforms, and is an index indicating the degree of hardness.

[0042] In order to reduce the temperature dependence of tan δ in the range from room temperature to high temperatures, the glass transition temperature of the adhesive sheet 11 is preferably -20°C or lower, more preferably -23°C or lower, and even more preferably -25°C or lower. The glass transition temperature is the temperature at which tan δ is maximized (peak top temperature). In the vicinity of the glass transition temperature, the temperature dependence of tan δ is large. When the glass transition temperature is sufficiently lower than the temperature of the device's operating environment, the temperature dependence of tan δ in the temperature range of the operating environment is reduced. In addition, when the glass transition temperature is in the above range, the adhesive sheet 11 has adhesive retention even in the low temperature range, so that peeling from the adherend at low temperatures is suppressed and the gap distance can be reduced.

[0043] The lower limit of the glass transition temperature of the adhesive sheet 11 is not particularly limited, but is generally −80° C. or higher. The glass transition temperature of the adhesive sheet 11 is preferably −70° C. or higher, more preferably −60° C. or higher, and may be −55° C. or higher, or −50° C. or higher. By setting the glass transition temperature of the adhesive sheet within the above range, it is possible to effectively increase the adhesive retention force and reduce the gap distance.

[0044] The thickness of the adhesive sheet 11 is not particularly limited, and may be appropriately adjusted depending on the thickness of the target device, the properties required for the adhesive sheet, and the like. From the viewpoint of increasing the adhesive strength of the adhesive sheet 11, the thickness is preferably 10 μm or more. From the viewpoint of providing cushioning against impacts from the outer surface, the thickness of the adhesive sheet 11 is preferably 25 μm or more, more preferably 30 μm or more, and may be 35 μm or more or 40 μm or more. From the viewpoint of thinning the device and suppressing the adhesive from protruding from the end surface during processing of the adhesive sheet and bending of the device, the thickness of the adhesive sheet 11 is preferably 100 μm or less, more preferably 75 μm or less.

[0045] The total light transmittance of the adhesive sheet 11 is preferably 85% or more, more preferably 90% or more, and even more preferably 91% or more. The haze of the adhesive sheet 11 is preferably 1.5% or less, more preferably 1% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less. In addition to the adhesive sheet 11, adhesive sheets arranged on the viewing side of the image display panels 51 and 54, such as the adhesive sheets 12 and 13, are preferably highly transparent, and the total light transmittance and haze are preferably within the above ranges.

[0046] The adhesive sheet 11 is not particularly limited in composition of the adhesive as long as the dielectric constant is within the above range, and examples of the adhesive include acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, and rubber-based polymers as the base polymer. As the adhesive, an acrylic adhesive containing an acrylic base polymer as the main component is preferable because transparency and adhesive strength can be controlled in addition to the dielectric constant and adhesive strength. The adhesive can be used alone or in combination of two or more kinds. The adhesive sheet formed by the adhesive may be in either a single layer form or a laminated form.

[0047] <Acrylic-based polymer> The acrylic base polymer contains an alkyl (meth)acrylate ester as a main constituent monomer component. In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0048] As the (meth)acrylic acid alkyl ester, there may be mentioned (meth)acrylic acid C having an alkyl group having 1 to 20 carbon atoms. 1-20 Alkyl esters are preferably used. The alkyl group in the (meth)acrylic acid alkyl ester may be either linear or cyclic. The alkyl group in the linear form (linear alkyl group) may be a straight-chain alkyl group or may have a branch.

[0049] Specific examples of the (meth)acrylic acid chain alkyl ester include C methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, and isononyl (meth)acrylate. 1-9 Chain alkyl esters; and C such as decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, and nonadecyl (meth)acrylate. 10-20 Examples of the alkyl ester include linear alkyl esters.

[0050] Specific examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group (cyclic alkyl group) include (meth)acrylic acid cycloalkyl esters such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; and (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring such as dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

[0051] In the acrylic base polymer, the amount of the (meth)acrylic acid alkyl ester is preferably 60 to 100 parts by weight, and more preferably 70 to 98 parts by weight, per 100 parts by weight of the total of the monomer components.

[0052] The acrylic base polymer is (meth)acrylic acid C as the (meth)acrylic acid alkyl ester. 10-20 It is preferable that the polymer contains a chain alkyl ester. When the acrylic base polymer contains a long-chain alkyl ester (meth)acrylic acid as a monomer component, the dipole moment of the molecule is reduced and the molar volume can be increased, so that the dielectric constant can be reduced. In addition, a homopolymer of an alkyl ester (meth)acrylic acid having a long-chain alkyl group having 10 or more carbon atoms has a temperature region (plateau region) above Tg where the temperature dependence of viscoelasticity is small. Therefore, when the base polymer contains a long-chain alkyl ester (meth)acrylic acid as a monomer component, the temperature dependence of tan δ can be reduced.

[0053] Since the temperature range of the plateau region is wide and the storage modulus in the plateau region is small, (meth)acrylic acid C 10-20 Among the chain alkyl esters, (meth)acrylic acid C 10-16 Alkyl esters are preferred, and (meth)acrylic acid C 10-13 Alkyl esters are more preferred. Among them, (meth)acrylic acid C 12 Alkyl esters are preferred, with dodecyl acrylate (lauryl acrylate) being particularly preferred.

[0054] Polymers of long-chain alkyl (meth)acrylate esters are characterized by a wide temperature range in the plateau region and a small storage modulus in the plateau region, but they are highly crystalline and have a high glass transition temperature. For example, the glass transition temperature of a homopolymer of lauryl acrylate is 0°C. In order to lower the glass transition temperature of the base polymer, it is necessary to use a monomer component containing (meth)acrylic acid C. 10-20 In addition to the chain alkyl ester, (meth)acrylic acid C 1-9It is preferable that the alkyl ester contains a chain alkyl ester.

[0055] (Meth)acrylic acid C 1-9 Among the chain alkyl esters, in order to lower the Tg of the base polymer, those having a homopolymer glass transition temperature of -40°C or lower are preferred. (Meth)acrylic acid C having a homopolymer glass transition temperature of -40°C or lower 1-9 Specific examples of chain alkyl esters include 2-ethylhexyl acrylate (Tg: -70°C), n-hexyl acrylate (Tg: -65°C), n-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), isooctyl acrylate (Tg: -58°C), butyl acrylate (Tg: -55°C), etc. Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred due to its low Tg.

[0056] In order to obtain a pressure sensitive adhesive having the above-mentioned properties, (meth)acrylic acid C is used as a monomer component of the acrylic base polymer. 10-20 Chain alkyl ester and (meth)acrylic acid C 1-9 It is preferable to contain both the linear alkyl ester and the chain alkyl ester and adjust the ratio of the two.

[0057] (Meth)acrylic acid C per 100 parts by weight of the total monomer components of the acrylic base polymer 10-20 The amount of the chain alkyl ester is preferably 5 to 55 parts by weight, more preferably 10 to 50 parts by weight, further preferably 15 to 45 parts by weight, and particularly preferably 20 to 50 parts by weight. In particular, the amount of lauryl acrylate is preferably within the above range. 1-9 The amount of the chain alkyl ester is preferably 30 to 80 parts by weight, more preferably 40 to 75 parts by weight, further preferably 45 to 70 parts by weight, and particularly preferably 50 to 65 parts by weight. In particular, the amount of 2-ethylhexyl acrylate is preferably within the above range.

[0058] The acrylic base polymer may contain a nitrogen-containing monomer as a monomer component. Examples of the nitrogen-containing monomer include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinyl carboxylic acid amides, and N-vinylcaprolactam, and cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred because it has a high effect of improving adhesive strength by improving cohesive strength.

[0059] A crosslinked structure may be introduced into the acrylic base polymer. By crosslinking the acrylic base polymer, even if the G' of the pressure-sensitive adhesive sheet is small, it is possible to exhibit high adhesive retention. In this way, in order to introduce a crosslinked structure into the acrylic base polymer, the acrylic base polymer preferably contains a hydroxyl group-containing monomer and a carboxyl group-containing monomer as monomer components in addition to the above-mentioned (meth)acrylic acid alkyl ester. When a crosslinked structure is introduced into the acrylic base polymer by an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, or the like, the hydroxyl group or the carboxyl group becomes the introduction point of the crosslinked structure.

[0060] Examples of the hydroxy group-containing monomer include (meth)acrylic acid esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate. Among these, 2-hydroxyethyl acrylate (Tg: -15°C) and 4-hydroxybutyl acrylate (Tg: -32°C) are preferred because they contribute greatly to improving the adhesive strength and can suppress clouding of the pressure-sensitive adhesive sheet 11 in a high humidity environment, and 4-hydroxybutyl acrylate is particularly preferred because of its low Tg.

[0061] Examples of the carboxy group-containing monomer include acrylic monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, as well as itaconic acid, maleic acid, fumaric acid, and crotonic acid.

[0062] From the viewpoint of increasing the adhesive strength and adhesive retention of the adhesive sheet 11, the amount of the polar group-containing monomer relative to a total of 100 parts by weight of the monomer components of the acrylic base polymer is preferably 2 parts by weight or more, and may be 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more. On the other hand, as the content of the polar monomer increases, the dipole moment of the base polymer increases and the relative dielectric constant increases. In addition, if the content of the polar monomer is excessively large, the glass transition temperature of the polymer increases, and the adhesive strength at low temperatures tends to decrease. Therefore, the amount of the polar group-containing monomer relative to a total of 100 parts by weight of the monomer components of the acrylic base polymer is preferably 15 parts by weight or less, and may be 13 parts by weight or less, or 10 parts by weight or less.

[0063] The acrylic base polymer preferably contains a hydroxy group-containing monomer and a nitrogen-containing monomer among the above polar monomer components, and the total amount of the hydroxy group-containing monomer and the nitrogen-containing monomer is preferably within the above range.

[0064] By including a hydroxyl group-containing monomer as the polar monomer component, the adhesive strength of the adhesive sheet 11 is improved and the opacity of the adhesive sheet 11 in a high humidity environment tends to be suppressed. Therefore, the amount of the hydroxyl group-containing monomer relative to a total of 100 parts by weight of the monomer components of the acrylic base polymer is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and may be 2 parts by weight or more. On the other hand, when the content of the hydroxyl group-containing monomer (the amount of hydroxyl groups in the adhesive) increases, the relative dielectric constant of the adhesive tends to increase significantly. Therefore, the amount of the hydroxyl group-containing monomer relative to a total of 100 parts by weight of the monomer components of the acrylic base polymer is preferably 10 parts by weight or less, more preferably 8 parts by weight or less, and may be 6 parts by weight or less. If the amount of the hydroxyl group-containing monomer is within the above range, the relative dielectric constant and the gap distance can be set within the above range.

[0065] From the viewpoint of achieving both improved adhesive strength and reduced dielectric constant, the amount of nitrogen-containing monomer relative to 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 0.5 to 10 parts by weight, more preferably 1 to 8 parts by weight. In particular, the amount of N-vinylpyrrolidone is preferably within the above range, since it contributes greatly to improving adhesive strength. If the amount of the nitrogen-containing monomer is within the above range, the relative dielectric constant and gap distance can be within the above range.

[0066] In order to prevent corrosion of the electrodes of the touch panel due to acid components, the adhesive sheet 11 preferably has a small acid content. In addition, in order to suppress polyenization of the polyvinyl alcohol polarizer due to the acid components, the adhesive sheet 11 preferably has a small acid content. In such an acid-free adhesive sheet, the content of organic acid monomers such as (meth)acrylic acid is preferably 100 ppm or less, more preferably 70 ppm or less, and even more preferably 50 ppm or less. The organic acid monomer content of the adhesive sheet 11 is determined by immersing the adhesive sheet in pure water, heating at 100°C for 45 minutes, and quantifying the acid monomers extracted into the water by ion chromatography.

[0067] In order to reduce the acid monomer content in the adhesive sheet 11, it is preferable that the amount of organic acid monomer components such as (meth)acrylic acid in the monomer components constituting the base polymer is small. Therefore, in order to make the adhesive sheet acid-free, it is preferable that the base polymer does not substantially contain organic acid monomers (carboxy group-containing monomers) as monomer components. In the acid-free adhesive sheet, the amount of carboxy group-containing monomers relative to a total of 100 parts by weight of the monomer components of the base polymer is preferably 0.5 parts by weight or less, more preferably 0.1 parts by weight or less, even more preferably 0.05 parts by weight or less, and ideally 0.

[0068] The acrylic base polymer may contain, as a monomer component, a monomer other than the above-mentioned (meth)acrylic acid alkyl ester and polar monomer. Examples of the monomer components other than the above include vinyl monomers such as (meth)acrylic acid caprolactone adducts, sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, vinyl acetate, vinyl propionate, styrene, and α-methylstyrene; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth)acrylate; glycol-based acrylic ester monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; acrylic ester monomers such as tetrahydrofurfuryl (meth)acrylate, fluorine (meth)acrylate, silicone (meth)acrylate, and 2-methoxyethyl (meth)acrylate.

[0069] The theoretical Tg of the acrylic base polymer is preferably -60 to -20°C. The theoretical Tg of the acrylic base polymer is more preferably -23°C or lower, further preferably -25°C or lower, and particularly preferably -30°C or lower. The theoretical Tg of the acrylic base polymer may be -50°C or lower, -45°C or lower, -40°C or lower, or -38°C or lower. If the theoretical Tg of the base polymer is within the above range, the relative dielectric constant and the gap distance can be set within the above range. The theoretical Tg is the glass transition temperature Tg of a homopolymer of the constituent monomer components of the acrylic base polymer. i and the weight fraction W of each monomer component i It is calculated using the following Fox formula: 1 / Tg=Σ(W i / Tg i )

[0070] Tg is the glass transition temperature of the polymer chain (unit: K), W i is the weight fraction of the monomer component i constituting the segment (copolymerization ratio by weight), Tg i is the glass transition temperature (unit: K) of a homopolymer of monomer component i. The glass transition temperature of a homopolymer can be determined from the values ​​listed in Polymer Handbook, 3rd Edition (John Wiley & Sons, Inc., 1989). For the Tg of a homopolymer of a monomer not listed in the above literature, the peak top temperature of tan δ measured by dynamic viscoelasticity measurement can be used.

[0071] <Cross-linked structure of base polymer> As described above, the acrylic base polymer may have a crosslinked structure. The introduction of a crosslinked structure into the base polymer increases the gel fraction of the adhesive. The gel fraction of the adhesive sheet 11 is preferably 55 to 85%, more preferably 60 to 80%, even more preferably 63 to 77%, and particularly preferably 65 to 75%. By adjusting the gel fraction within this range, even when G' is small and the adhesive sheet is soft, high adhesive retention can be exerted, and the gap distance can be reduced.

[0072] The gel fraction can be determined as the insoluble matter in a solvent such as ethyl acetate, specifically, the weight fraction (unit: weight %) of the insoluble matter after immersing the pressure-sensitive adhesive sheet in ethyl acetate for 7 days at 23°C relative to the sample before immersion. In general, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked parts in the polymer, the higher the gel fraction. The gel fraction (amount of crosslinked structure introduced) can be adjusted to a desired range by the method of introducing the crosslinked structure, the type and amount of the crosslinking agent, etc.

[0073] Methods for introducing a crosslinked structure into a base polymer include (1) a method in which a base polymer having a functional group capable of reacting with a crosslinking agent is polymerized, and then a crosslinking agent is added to react the base polymer with the crosslinking agent; and (2) a method in which a multifunctional compound is included in the polymerization components of the base polymer to introduce a branched structure (crosslinked structure) into the polymer chain. These methods may be used in combination to introduce multiple types of crosslinked structures into the base polymer.

[0074] In the above method (1) of reacting a base polymer with a crosslinking agent, a crosslinking agent is added to the polymerized base polymer and heated as necessary to introduce a crosslinked structure into the base polymer. Examples of the crosslinking agent include compounds that react with functional groups such as hydroxyl groups and carboxyl groups contained in the base polymer. Specific examples of the crosslinking agent include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents.

[0075] Among them, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they are highly reactive with the hydroxyl and carboxyl groups of the base polymer and can easily introduce a crosslinked structure. These crosslinking agents react with functional groups such as hydroxyl and carboxyl groups introduced into the base polymer to form a crosslinked structure. In an acid-free adhesive in which the base polymer does not contain a carboxyl group, it is preferred to use an isocyanate-based crosslinking agent to form a crosslinked structure by reacting the hydroxyl groups in the base polymer with the isocyanate crosslinking agent.

[0076] As the isocyanate crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate crosslinking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trimethylolpropane / Examples of isocyanate adducts include tolylene diisocyanate trimer adducts (e.g., "Coronate L" manufactured by Tosoh), trimethylolpropane / hexamethylene diisocyanate trimer adducts (e.g., "Coronate HL" manufactured by Tosoh), trimethylolpropane adducts of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals, and isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh).

[0077] In the above method (2) of including a polyfunctional monomer in the polymerization components of the base polymer, the monomer components constituting the acrylic base polymer and the polyfunctional compound for introducing a crosslinking structure may be reacted in their entirety at once, or the polymerization may be carried out in multiple stages. As a method of carrying out polymerization in multiple stages, a method is preferred in which a monofunctional monomer constituting the base polymer is polymerized (preliminary polymerization) to prepare a partial polymer (prepolymer composition), and a polyfunctional compound such as a polyfunctional (meth)acrylate is added to the prepolymer composition to polymerize the prepolymer composition and the polyfunctional monomer (main polymerization). The prepolymer composition is a partial polymer containing a polymer with a low degree of polymerization and an unreacted monomer.

[0078] By carrying out prepolymerization of the components of the acrylic base polymer, branching points (crosslinking points) due to the polyfunctional compound can be uniformly introduced into the base polymer. In addition, a mixture (adhesive composition) of a low molecular weight polymer or a partially polymerized product and an unpolymerized monomer component can be applied onto a substrate, and then main polymerization can be carried out on the substrate to form a pressure-sensitive adhesive sheet. Since low-polymer compositions such as prepolymer compositions have low viscosity and excellent applicability, the method of carrying out main polymerization on a substrate after applying an adhesive composition, which is a mixture of a prepolymer composition and a polyfunctional compound, can improve the productivity of the pressure-sensitive adhesive sheet and make the thickness of the pressure-sensitive adhesive sheet uniform.

[0079] The polyfunctional compound used for introducing the crosslinked structure may be a compound containing two or more polymerizable functional groups (ethylenically unsaturated groups) having an unsaturated double bond in one molecule. As the polyfunctional compound, a polyfunctional (meth)acrylate is preferred because it is easily copolymerized with the monomer component of the acrylic base polymer. When introducing a branched (crosslinked) structure by active energy ray polymerization (photopolymerization), a polyfunctional acrylate is preferred.

[0080] Examples of polyfunctional (meth)acrylates include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol dimethacrylate. Examples of the di(meth)acrylate include pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, and isoprene (meth)acrylate.

[0081] The molecular weight of the polyfunctional compound such as a polyfunctional (meth)acrylate is preferably 1500 or less, and more preferably 1000 or less. The functional group equivalent (g / eq) of the polyfunctional compound is preferably 50 to 500, more preferably 70 to 300, and even more preferably 80 to 200. When the molecular weight of the polyfunctional compound is within this range, the gap distance can be reduced.

[0082] <Preparation of Base Polymer> The acrylic base polymer can be prepared by a known polymerization method such as solution polymerization, UV polymerization, bulk polymerization, emulsion polymerization, etc. In terms of the transparency, water resistance, cost, etc. of the adhesive, the solution polymerization method or the active energy ray polymerization method (e.g., UV polymerization) is preferred. As the solvent for solution polymerization, ethyl acetate, toluene, etc. are generally used.

[0083] In preparing the acrylic base polymer, a polymerization initiator such as a photopolymerization initiator or a thermal polymerization initiator may be used depending on the type of polymerization reaction. The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization, and for example, a benzoin ether-based photopolymerization initiator, an acetophenone-based photopolymerization initiator, an α-ketol-based photopolymerization initiator, an aromatic sulfonyl chloride-based photopolymerization initiator, a photoactive oxime-based photopolymerization initiator, a benzoin-based photopolymerization initiator, a benzyl-based photopolymerization initiator, a benzophenone-based photopolymerization initiator, a ketal-based photopolymerization initiator, a thioxanthone-based photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, etc. can be used. As the thermal polymerization initiator, for example, an azo-based initiator, a peroxide-based initiator, or a redox-based initiator combining a peroxide and a reducing agent (for example, a combination of a persulfate and sodium hydrogen sulfite, a combination of a peroxide and sodium ascorbate, etc.) can be used.

[0084] In the polymerization, a chain transfer agent, a polymerization inhibitor (polymerization retarder), etc. may be used for the purpose of molecular weight adjustment, etc. Examples of the chain transfer agent include thiols such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol, and α-methylstyrene dimer.

[0085] The molecular weight of the base polymer can be adjusted by adjusting the type and amount of the polymerization initiator. For example, in radical polymerization, the more the amount of polymerization initiator, the higher the radical concentration in the reaction system, so the density of reaction initiation points is high and the molecular weight tends to be small. Conversely, the less the amount of polymerization initiator, the lower the density of reaction initiation points, so the polymer chains tend to extend more easily and the molecular weight tends to be large.

[0086] In order to obtain a pressure sensitive adhesive sheet with excellent adhesive strength and small gap distance, it is preferable that the acrylic base polymer has a high gel fraction at a low crosslinking density. In order to increase the gel fraction (the ratio of polymer chains with crosslinked structures) at a low crosslinking density, the molecular weight (length of polymer chain) of the base polymer should be increased. As mentioned above, in order to increase the molecular weight of the base polymer, it is preferable to reduce the amount of polymerization initiator used when polymerizing the base polymer.

[0087] The amount of the polymerization initiator used during polymerization of the base polymer may be appropriately set depending on the type of polymerization reaction, the monomer composition, the type of polymerization initiator, the target molecular weight, etc. From the viewpoint of increasing the molecular weight of the base polymer and increasing the gel fraction with a small amount of crosslinking agent, the amount of the polymerization initiator used is preferably 0.001 to 0.4 parts by weight, more preferably 0.003 to 0.1 parts by weight, and even more preferably 0.005 to 0.05 parts by weight, relative to 100 parts by weight of the total of the monomer components constituting the base polymer.

[0088] When a crosslinking structure is introduced by an isocyanate-based crosslinking agent, it is preferable to polymerize the base polymer by solution polymerization, add the crosslinking agent, and heat as necessary to introduce a crosslinking structure into the base polymer.When a crosslinking structure is introduced by a multifunctional compound such as a multifunctional (meth)acrylate, it is preferable to polymerize the base polymer or prepare a prepolymer composition by solution polymerization or active energy ray polymerization, add the multifunctional compound, and then introduce a crosslinking structure by the multifunctional compound by active energy ray polymerization.

[0089] The prepolymer composition can be prepared, for example, by partially polymerizing (preliminarily polymerizing) a composition (referred to as a "prepolymer-forming composition") obtained by mixing a monomer component constituting an acrylic-based polymer with a polymerization initiator. The monomer in the prepolymer-forming composition is preferably a monofunctional monomer component such as a (meth)acrylic acid alkyl ester or a polar group-containing monomer. The prepolymer-forming composition may contain a polyfunctional monomer in addition to the monofunctional monomer. For example, a part of the polyfunctional monomer may be contained in the prepolymer-forming composition, and the remainder of the polyfunctional monomer component may be added after the preliminarily polymerized to carry out the main polymerization.

[0090] The polymerization rate of the prepolymer is not particularly limited, but is preferably 3 to 50% by weight, more preferably 5 to 40% by weight, from the viewpoint of obtaining a viscosity suitable for application onto a substrate. The polymerization rate of the prepolymer can be adjusted to a desired range by adjusting the type and amount of the photopolymerization initiator, the irradiation intensity and irradiation time of the actinic ray such as UV light, etc.

[0091] <Acrylic oligomer> The adhesive sheet 11 may contain an oligomer in addition to the acrylic base polymer. The acrylic oligomer has a weight average molecular weight of about 1000 to 30000. The acrylic oligomer contains an alkyl (meth)acrylate ester as a main constituent monomer component.

[0092] The glass transition temperature of the acrylic oligomer is preferably 60°C or higher, more preferably 80°C or higher, even more preferably 100°C or higher, and particularly preferably 110°C or higher. By using a low Tg acrylic base polymer with a crosslinked structure and a high Tg acrylic oligomer in combination, the adhesive strength of the pressure-sensitive adhesive sheet, especially the adhesive retention at high temperatures, tends to improve, and the gap distance can be reduced. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is generally 200°C or lower, preferably 180°C or lower, and more preferably 160°C or lower. The glass transition temperature of the acrylic oligomer is calculated by the above-mentioned Fox formula.

[0093] The acrylic oligomer having a glass transition temperature of 60° C. or higher preferably contains, as a constituent monomer component, a (meth)acrylic acid alkyl ester having a chain alkyl group (chain alkyl (meth)acrylate) and a (meth)acrylic acid alkyl ester having an alicyclic alkyl group (alicyclic alkyl (meth)acrylate). Specific examples of the chain alkyl (meth)acrylate and the alicyclic alkyl (meth)acrylate are as exemplified above as the constituent monomers of the acrylic polymer chain.

[0094] Among the exemplified (meth)acrylic acid alkyl esters, methyl methacrylate is preferred as the chain alkyl (meth)acrylate because it has a high glass transition temperature and excellent compatibility with the base polymer. As the alicyclic alkyl (meth)acrylate, dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred. That is, the acrylic oligomer preferably contains, as a constituent monomer component, one or more selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate.

[0095] The amount of the alicyclic alkyl (meth)acrylate relative to the total amount of the monomer components constituting the acrylic oligomer is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and even more preferably 30 to 70% by weight. The amount of the chain alkyl (meth)acrylate relative to the total amount of the monomer components constituting the acrylic oligomer is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and even more preferably 30 to 70% by weight.

[0096] The weight average molecular weight of the acrylic oligomer is preferably 1000 to 30000, more preferably 1500 to 10000, and further preferably 2000 to 8000. By using an acrylic oligomer having a molecular weight within this range, the adhesive strength and adhesive retention strength of the pressure-sensitive adhesive tend to be improved, and the gap distance can be reduced.

[0097] The acrylic oligomer can be obtained by polymerizing the above-mentioned monomer components by various polymerization methods. Various polymerization initiators may be used in the polymerization of the acrylic oligomer. In addition, a chain transfer agent may be used for the purpose of adjusting the molecular weight.

[0098] The content of the acrylic oligomer in the adhesive sheet 11 is not particularly limited, but in order to sufficiently increase the adhesive strength, the amount of the acrylic oligomer relative to 100 parts by weight of the base polymer is preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, and even more preferably 1 part by weight or more. The amount of the acrylic oligomer in the adhesive sheet 11 may be 1.3 parts by weight or more, 1.5 parts by weight or more, 1.8 parts by weight or more, 2 parts by weight or more, 2.3 parts by weight or more, or 2.5 parts by weight or more relative to 100 parts by weight of the base polymer. The greater the amount of the high Tg acrylic oligomer added, the smaller the gap distance tends to be.

[0099] On the other hand, if the amount of the acrylic oligomer added is excessively large, the haze of the adhesive sheet increases due to a decrease in compatibility, and the transparency tends to decrease. Since high transparency is required for the adhesive sheet arranged on the viewing side of the image display panel, the amount of the acrylic oligomer in the adhesive sheet 11 is preferably 5 parts by weight or less, and may be 4 parts by weight or less, or 3 parts by weight or less, relative to 100 parts by weight of the base polymer.

[0100] <Adhesive composition> The acrylic base polymer (or prepolymer composition) is mixed with the above-mentioned acrylic oligomer, a crosslinking agent and / or a polyfunctional compound for introducing a crosslinked structure, and other additives, as necessary, to prepare a pressure-sensitive adhesive composition. The remaining monomer components constituting the acrylic base polymer may be added to the pressure-sensitive adhesive composition as necessary. A thickening additive may be used for the purpose of viscosity adjustment, etc.

[0101] When the pressure-sensitive adhesive composition contains a prepolymer composition and a polyfunctional compound, the pressure-sensitive adhesive composition preferably contains a photopolymerization initiator for the main polymerization. After the preliminary polymerization, the polymerization initiator for the main polymerization may be added to the prepolymer composition. When the polymerization initiator from the preliminary polymerization remains in the prepolymer composition without being deactivated, the addition of the polymerization initiator for the main polymerization may be omitted. The pressure-sensitive adhesive composition may contain a chain transfer agent.

[0102] The pressure-sensitive adhesive composition preferably has an acrylic base polymer (or prepolymer composition) content of 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, relative to the total non-volatile content.

[0103] The amount of the crosslinking agent and / or the polyfunctional compound in the pressure-sensitive adhesive composition may be adjusted so that the gel fraction falls within the above range. As described above, in order to reduce the gap distance, it is preferable to increase the molecular weight of the acrylic base polymer and increase the gel fraction at a low crosslinking point density. For example, when a crosslinking structure is introduced by an isocyanate crosslinking agent, the amount of the crosslinking agent is preferably 0.005 to 0.5 parts by weight, more preferably 0.01 to 0.3 parts by weight, and even more preferably 0.02 to 0.1 parts by weight, relative to 100 parts by weight of the acrylic base polymer. When a crosslinking structure is introduced by a polyfunctional (meth)acrylate, the amount of the polyfunctional (meth)acrylate is preferably 0.005 to 0.3 parts by weight, more preferably 0.01 to 0.2 parts by weight, and even more preferably 0.02 to 0.1 parts by weight, relative to 100 parts by weight of the acrylic base polymer (prepolymer).

[0104] (Silane coupling agent) A silane coupling agent may be added to the pressure-sensitive adhesive composition. When a silane coupling agent is added to the pressure-sensitive adhesive composition, the amount added is usually about 0.01 to 5.0 parts by weight, and preferably about 0.03 to 3.0 parts by weight, based on 100 parts by weight of the base polymer. When the amount of the silane coupling agent is within the above range, the gap distance may be reduced.

[0105] (Other additives) In addition to the above-exemplified components, the pressure-sensitive adhesive composition may contain additives such as a tackifier, a plasticizer, a softener, an anti-deterioration agent, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, and an antistatic agent.

[0106] <Formation of adhesive sheet> The adhesive composition is applied onto a substrate, and the solvent is dried and removed as necessary, and / or the main polymerization is performed by irradiation with active light rays, to form an adhesive sheet on the substrate. Any suitable substrate is used as the substrate used to form the adhesive sheet. The substrate may be a release film having a release layer on the surface that contacts the adhesive sheet.

[0107] As the film substrate of the release film, films made of various resin materials are used. Examples of the resin materials include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, acetate resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, and polyphenylene sulfide resins. Among these, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10 to 200 μm, and more preferably 25 to 150 μm. Examples of materials for the release layer include silicone-based release agents, fluorine-based release agents, long-chain alkyl-based release agents, and fatty acid amide-based release agents. The thickness of the release layer is generally about 10 to 2000 nm.

[0108] As a method for applying the pressure-sensitive adhesive composition onto a substrate, various methods such as roll coating, kiss roll coating, gravure coating, reverse coating, roll brush, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, and die coater can be used.

[0109] When the base polymer of the pressure-sensitive adhesive composition is a solution polymer, it is preferable to dry the solvent after application. As the drying method, a suitable method can be adopted depending on the purpose. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and particularly preferably 70°C to 170°C. As the drying time, a suitable time can be adopted. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and particularly preferably 10 seconds to 10 minutes.

[0110] When the pressure-sensitive adhesive composition contains a crosslinking agent, a crosslinking reaction may be carried out after the pressure-sensitive adhesive composition is applied onto the substrate. During crosslinking, heating may be carried out as necessary. The temperature of the crosslinking reaction is usually in the range of 20°C to 160°C, and the time of the crosslinking reaction is about 1 minute to 7 days. After the pressure-sensitive adhesive composition is applied, heating for drying the solvent may also serve as heating for crosslinking. After drying the solvent, a cover sheet is preferably attached to protect the surface of the pressure-sensitive adhesive sheet. As the cover sheet, it is preferable to use a release film having a release layer on the surface in contact with the pressure-sensitive adhesive sheet, similar to the substrate film.

[0111] When the pressure-sensitive adhesive composition is a photopolymerizable composition containing a prepolymer composition and a polyfunctional compound, the pressure-sensitive adhesive composition is applied in a layer on a substrate, and then photocured by irradiating with active light. When photocuring, it is preferable to attach a cover sheet to the surface of the coating layer, and irradiate with active light while the pressure-sensitive adhesive composition is sandwiched between the two sheets, thereby preventing polymerization inhibition due to oxygen.

[0112] The actinic rays may be selected according to the type of polymerizable components such as monomers and polyfunctional (meth)acrylates, the type of photopolymerization initiator, etc., and generally, ultraviolet light and / or short-wavelength visible light is used. The cumulative light amount of the irradiated light is 100 to 5000 mJ / cm. 2The light source for light irradiation is not particularly limited as long as it can irradiate light within a wavelength range to which the photopolymerization initiator contained in the pressure-sensitive adhesive composition has sensitivity, and preferably used are an LED light source, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, a xenon lamp, etc.

[0113] [Second adhesive sheet] The composition of the second adhesive sheet 12 arranged on the image display panel side surface of the polarizing plate 31 is not particularly limited as long as it has the above-mentioned relative dielectric constant. The second adhesive sheet 12 is not required to have the same impact resistance as the first adhesive sheet 11 arranged on the viewing side. From the viewpoint of thinning, the thickness of the second adhesive sheet 12 is preferably smaller than the thickness of the first adhesive sheet 11. The thickness of the second adhesive sheet 12 is preferably 10 to 25 μm, and may be 20 μm or less.

[0114] As long as the dielectric constant of the second adhesive sheet 12 is within the above range, the composition of the adhesive is not particularly limited, and examples of the base polymer include acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, rubber, and other polymers.

[0115] The second adhesive sheet 12 is preferably composed of an acrylic adhesive containing an acrylic base polymer, since this allows control of transparency, adhesive strength, etc. in addition to the dielectric constant and adhesive strength, etc. As described above, in order to reduce the dielectric constant of the adhesive sheet, it is effective to reduce the amount of polar group-containing monomer in the monomer components that make up the base polymer.

[0116] When the second adhesive sheet 12 is an acrylic adhesive sheet, the amount of the polar group-containing monomer relative to a total of 100 parts by weight of the monomer components of the base polymer is preferably 15 parts by weight or less, more preferably 10 parts by weight or less, and may be 5 parts by weight or less or 3 parts by weight or less. The amount of the hydroxyl group-containing monomer relative to a total of 100 parts by weight of the monomer components of the base polymer is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, and may be 3 parts by weight or less.

[0117] The second adhesive sheet 12 is not required to have the same strain relaxation property as the first adhesive sheet 11. Therefore, the storage modulus G' of the second adhesive sheet 12 at 25°C 25 is G' of the first adhesive sheet 25 The G' of the second adhesive sheet 12 may be larger than 25 The G' of the second adhesive sheet 12 may be 70 kPa or more, 75 kPa or more, or 80 kPa or more. 25 By increasing the bonding strength, the adhesive holding power can be improved.

[0118] Similarly to the first adhesive sheet 11, the second adhesive sheet 12 also preferably has a gap distance of 2 mm or less. By having a gap distance of 2 mm or less and the above-mentioned relative dielectric constant, the reliability can be improved even in a foldable image display device having a touch panel within a distance of 500 μm from the touch surface. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is little or no malfunction of the touch panel sensor, and failures and the like are suppressed or prevented, resulting in high reliability.

[0119] [Image display device] As described above, the first adhesive sheet 11 and the second adhesive sheet 12 are used to bond the polarizing plate 31 to other optical members in a foldable image display device equipped with a touch panel.

[0120] In the image display device 101 shown in Fig. 1, a touch panel 41, a circular polarizer 31, and a cover window 71 are arranged on the viewing side surface of an organic EL panel 51 serving as an image display panel. In a flexible display, all of these members are flexible and bendable. In the image display device 102 shown in Fig. 2, a touch panel is integrated with an organic EL panel 54 serving as an image display panel, and the circular polarizer 31 and the cover window 71 are arranged on the surface of the organic EL panel 54.

[0121] <Image display panel> The organic EL panel includes a pair of electrodes and an organic light-emitting layer sandwiched between the electrodes on a substrate. The organic EL panel may be either a top-emission type in which a metal electrode, an organic light-emitting layer, and a transparent electrode are laminated in this order on a substrate, or a bottom-emission type in which a transparent electrode, an organic light-emitting layer, and a metal electrode are laminated in this order on a transparent substrate. In both the bottom-emission type and the top-emission type, the substrate and sealing member provided on the viewing side of the organic light-emitting layer are transparent. The substrate and sealing member provided on the back side of the organic light-emitting layer (the housing 75 side in Figs. 1 and 2) do not have to be transparent. In the bottom-emission type flexible organic EL panel, the substrate does not have to be transparent, and polyimide or the like may be used as the substrate material. The substrate material may be a transparent resin material such as polyether ether ketone or transparent polyimide. A back sheet may be provided on the back side of the substrate for the purpose of protecting and reinforcing the substrate.

[0122] The image display panel is not limited to an organic EL panel, and may be a liquid crystal panel, an electrophoretic display panel (electronic paper), etc. For example, a bendable liquid crystal panel can be formed by using flexible substrates such as resin substrates as transparent substrates that sandwich the liquid crystal layer.

[0123] <Cover window> A cover window 71 is provided on the outermost surface on the viewing side of the image display device for the purpose of preventing damage to the image display panel due to impact from the outer surface. In a flexible display, a flexible transparent substrate such as transparent polyimide, polyether ether ketone, or polyethylene terephthalate is used as the cover window 71. A flexible glass plate (glass film) may be used as the material of the cover window 71, and the cover window 71 may have a laminated structure of a glass film and a resin film. From the viewpoint of achieving both strength and bendability, the thickness of the cover window is preferably 20 to 300 μm, more preferably 25 to 250 μm, and even more preferably 30 to 200 μm. In order to provide excellent recovery after maintaining a bent state for a long period of time, the yield point elongation of the cover window is preferably 5% or more. A bendable thin glass substrate may be used as the cover window 71. The cover window may be a laminate of two or more layers of transparent materials. An antireflection layer, a hard coat layer, or the like may be provided on the viewing side surface of the cover window.

[0124] <Touch panel> The image display device includes a capacitive touch panel on the viewing side surface of the image display panel. A capacitive touch panel detects a touch position based on a change in the amount of electricity when an operator's finger, a touch pen, or the like touches the touch surface. In the configuration of Fig. 1, a touch panel 41 is disposed between a circular polarizer 31 and an organic EL panel 51. In the configuration of Fig. 2, a touch panel is provided inside an image display panel 54. The touch panel may be disposed between the circular polarizer 31 and a cover window 71.

[0125] <Polarizing plate> Polarizing plate 31 is disposed on the viewing side of the image display panel. For example, in a liquid crystal display device, the polarizing plate provided on the viewing side of the liquid crystal panel adjusts the transmittance according to the polarization state of light transmitted through the liquid crystal cell. In an organic EL display device, by providing circular polarizing plate 31 on the viewing side of organic EL panel 51, it is possible to block external light reflected by the metal electrode of the organic EL panel from being emitted to the viewing side, thereby improving the visibility of the display.

[0126] The polarizing plate is generally made of a polarizer with an appropriate transparent protective film attached to one or both sides as required. The polarizer is not particularly limited, and various types can be used. Examples of the polarizer include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and partially saponified ethylene-vinyl acetate copolymer films, which are uniaxially stretched after adsorbing dichroic substances such as iodine or dichroic dyes, and polyene-based oriented films such as dehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride.

[0127] A thin polarizer having a thickness of 10 μm or less can also be used as the polarizer. Examples of the thin polarizer include polarizers described in JP-A-51-069644, JP-A-2000-338329, WO2010 / 100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. The thin polarizer can be obtained by a manufacturing method including, for example, a step of stretching a polyvinyl alcohol-based resin layer and a resin substrate for stretching in a laminated state, and a step of dyeing with a dichroic material such as iodine.

[0128] As the transparent protective film for the polarizer, a film having excellent transparency, mechanical strength, thermal stability, moisture blocking property, and optical isotropy, such as a cellulose-based resin, a cyclic polyolefin-based resin, an acrylic-based resin, a phenylmaleimide-based resin, a polycarbonate-based resin, etc. When transparent protective films are provided on both sides of the polarizer, protective films made of the same polymer material may be used on the front and back of the polarizer, or protective films made of different polymer materials may be used.

[0129] An optical film may be laminated on one or both surfaces of the polarizing plate, if necessary, via an appropriate adhesive layer or pressure-sensitive adhesive layer. As such a film, a retardation plate, a viewing angle widening film, a viewing angle limiting (peeping prevention) film, a brightness improving film, etc., which are used for forming an image display device, are used, and the type is not particularly limited. For example, in a liquid crystal display device, an optical compensation film may be used between an image display panel (liquid crystal panel) and a polarizing plate for the purpose of appropriately converting the polarization state of light emitted from a liquid crystal cell to the viewing side to improve viewing angle characteristics.

[0130] As described above, in an organic EL display device, by providing a circular polarizing plate in which a quarter-wave plate is arranged on the surface of the polarizer facing the organic EL panel, it is possible to block the emission of external light reflected by the metal electrode to the viewing side. By arranging a quarter-wave plate on the viewing side of the polarizer and making the emitted light circularly polarized, it is possible to allow a viewer wearing polarized sunglasses to view an appropriate image display. These optical films (optically anisotropic films) may be laminated on the polarizer without any other film in between. In this case, the optical film also functions as a protective film for the polarizer.

[0131] The thickness of the polarizing plate is generally about 10 to 200 μm. From the viewpoint of providing flexibility, the thickness of the polarizing plate used in a flexible display is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less. When optical films such as a quarter-wave plate are laminated on the polarizing plate, the total thickness including these films is preferably within the above range.

[0132] <Lamination between components using adhesive sheets> An adhesive sheet is used to bond the flexible members together. In the image display device shown in Fig. 1, the organic EL panel 51 and the bottom surface of the housing 75 are bonded together via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded together via an adhesive sheet 13, the touch panel 41 and the circular polarizer 31 are bonded together via a second adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via a first adhesive sheet 11. In the image display device shown in Fig. 2, the touch panel integrated organic EL panel 54 and the circular polarizer 31 are bonded together via the second adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via the first adhesive sheet 11.

[0133] Since both the second adhesive sheet 12 and the first adhesive sheet 11, which are arranged on the viewing side of the touch panel 41, have a low dielectric constant, malfunction of the touch panel can be reduced even if the distance D from the touch surface to the touch panel is small.

[0134] When the image display device of the present invention is subjected to a 240-hour bending retention test in a high-temperature and high-humidity environment at a temperature of 60° and a relative humidity of 95% in a bent state with a bending radius of 1.3 mm and a bending angle of 180°, it is preferable that peeling between members at the bent portion is small, and in particular, peeling at the interface between the polarizing plate 31 and the cover window 71 is small. The peeling at the interface in the bending retention test can be quantified as the length of the gap in the short side direction along the bending axis (gap distance) when a bending retention test is performed on a sample of 35 mm×100 mm in size with the short side direction as the bending axis. The gap distance of the image display device after the bending retention test is preferably 2 mm or less, preferably 1.5 mm or less, more preferably 1.0 mm or less, and may be 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. The lower limit of the gap distance is not particularly limited, and may be 0.

[0135] As described above, the gap distance of the image display device can be reduced by adjusting the composition of the first adhesive sheet 11. The gap distance of the adhesive sheet can also be evaluated by the same method as above. Specifically, the length of the gap (gap distance) between the adhesive sheet and the adherend may be measured by the following steps A to D.

[0136] Step A: Prepare a 35mm x 100mm test piece by bonding the adhesive sheet to the substrate. Step B: The test piece prepared in step A is bent along the short side with a bending radius of 1.3 mm and a bending angle of 180°. Step C: The test piece bent in step B is kept bent for 240 hours in an environment with a temperature of 60 degrees and a relative humidity of 95%. Step D: After maintaining the test piece in step C for 240 hours, measure the length of the gap between the pressure-sensitive adhesive sheet and the adherend in the short side direction at the bent portion of the test piece.

[0137] Peeling (gaps) in the bending retention test is likely to occur from the end of the bending axis of the test piece (end in the short side direction). When gaps occur from both ends or when there are gaps in multiple places, the length of the gap with the longest length in the short side direction is taken as the gap distance. When there are gaps in multiple places, the longest gap may be 2 mm or less. Preferably, the total length of each gap is 2 mm or less, and the total length of the gaps may be 1.5 mm or less, 1.0 mm or less, 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less, or may be 0.

[0138] In forming the image display device, the order in which each component is bonded is not particularly limited. The touch panel 41, the circular polarizer 31 and the cover window 71 may be laminated in that order on the image display panel 51, or a laminate in which two or more components are previously laminated via an adhesive sheet may be bonded on the image display panel 51. EXAMPLES

[0139] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0140] [Example 1] <First Adhesive Sheet> (Preparation of Acrylic Oligomer) 60 parts by weight of dicyclopentanyl methacrylate (DCPMA) and 40 parts by weight of methyl methacrylate (MMA) as monomer components, 3.5 parts by weight of α-thioglycerol as a chain transfer agent, and 100 parts by weight of toluene as a polymerization solvent were mixed and stirred at 70°C for 1 hour under a nitrogen atmosphere. Next, 0.2 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) was added as a thermal polymerization initiator and reacted at 70°C for 2 hours, and then the temperature was raised to 80°C and reacted for 2 hours. Thereafter, the reaction liquid was heated to 130°C, and toluene, the chain transfer agent, and unreacted monomers were dried and removed to obtain a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer was 5100, and the glass transition temperature (Tg) was 130°C.

[0141] (Polymerization of prepolymer) As monomer components for forming a prepolymer, 43 parts by weight of lauryl acrylate (LA), 44 parts by weight of 2-ethylhexyl acrylate (2EHA), 6 parts by weight of 4-hydroxybutyl acrylate (4HBA), and 7 parts by weight of N-vinyl-2-pyrrolidone (NVP) were mixed, and 0.015 parts by weight of "Omnirad 184" manufactured by IGM Resins was used as a photopolymerization initiator. The mixture was irradiated with ultraviolet light to polymerize, and a prepolymer composition (polymerization rate: approximately 10%) was obtained.

[0142] (Preparation of Pressure-Sensitive Adhesive Composition) To 100 parts by weight of the above prepolymer composition, 0.07 parts by weight of 1,6-hexanediol diacrylate (HDDA), 3 parts by weight of the above oligomer, and 0.3 parts by weight of a silane coupling agent ("KBM403" manufactured by Shin-Etsu Chemical Co., Ltd.) were added as post-added components, and then these were mixed uniformly to prepare a pressure-sensitive adhesive composition.

[0143] (Preparation of adhesive sheet) A 75 μm thick polyethylene terephthalate (PET) film ("Diafoil MRF75" manufactured by Mitsubishi Chemical) with a silicone-based release layer on the surface was used as a substrate (doubles as a heavy-duty release film), and the above-mentioned photocurable adhesive composition was applied to the substrate to a thickness of 50 μm to form a coating layer. A 75 μm thick PET film ("Diafoil MRE75" manufactured by Mitsubishi Chemical) with one side treated with silicone release was attached as a cover sheet (doubles as a light-duty release film) to this coating layer. This laminate was laminated from the cover sheet side with an irradiation intensity of 5 mW / cm2 on the irradiation surface directly under the lamp. 2 The adhesive sheet was then photocured by irradiating it with ultraviolet light using a black light that was adjusted in position so that the adhesive sheet was 50 μm thick.

[0144] <Second adhesive sheet> In a reaction vessel, 99 parts by weight of butyl acrylate (BA) and 1 part by weight of 4-hydroxybutyl acrylate (4HBA) as monomers, and 0.3 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator were charged together with ethyl acetate, and reacted for 4 hours at 60°C under a nitrogen gas stream. Ethyl acetate was then added to the reaction solution to obtain a solution of an acrylic polymer having a weight average molecular weight of 1.65 million. This solution was mixed with 0.3 parts by weight of dibenzoyl peroxide (manufactured by Nippon Oil & Fats Co., Ltd.'s "Niper BMT") and 0.1 parts by weight of trimethylolpropane xylylene diisocyanate (manufactured by Mitsui Chemicals Co., Ltd.'s "Takenate D110N") as crosslinking agents, and 3 parts by weight of a silane coupling agent (manufactured by Shin-Etsu Chemical Co., Ltd.'s "KBM403"), per 100 parts by weight of the polymer, to obtain a pressure-sensitive adhesive composition A.

[0145] The above adhesive composition A was applied to the release-treated surface of a 38 μm-thick PET film ("MRF38" manufactured by Mitsubishi Chemical) with a silicone-based release layer on its surface, and dried and crosslinked at 150°C to obtain an adhesive sheet with a thickness of 15 μm.

[0146] <Preparation of specimens for bending retention test> The release film on one side of the first adhesive sheet was peeled off, and a 51 μm thick polarizing plate was attached using a 2 kg roller. The release film on the other side of the adhesive sheet was peeled off, and a 80 μm thick transparent polyimide film was attached using a 2 kg roller. Furthermore, a 125 μm thick PET film was attached to the polarizing plate via a second adhesive sheet using a 2 kg roller. When attaching, plasma treatment was performed on the surfaces of the polarizing plate, the polyimide film, and the PET film before attaching them to the adhesive sheet.

[0147] This laminate was cut into a 35 mm×100 mm rectangle so that the absorption axis direction of the polarizing plate was parallel to the long side direction, and autoclaved at 35° C. and 0.35 MPa for 15 minutes to obtain a sample for evaluation.

[0148] [Examples 2 to 5, Comparative Examples 1 to 3] In producing the first adhesive sheet, the monomer composition charged in the polymerization of the prepolymer, the blending amount of the polyfunctional monomer (HDDA), and the blending amount of the oligomer were changed as shown in Table 1. Otherwise, a photocurable adhesive composition was prepared in the same manner as in Example 1, and then coated on a substrate and photocured to obtain a first adhesive sheet. Using the obtained first adhesive sheet, an evaluation sample was produced in the same manner as in Example 1, in which a polyimide film, a first adhesive sheet, a polarizing plate, a second adhesive sheet, and a PET film were laminated in this order.

[0149] [Evaluation of First Adhesive Sheet] <Gel fraction> About 0.2 g of adhesive was scraped off from the adhesive sheet, wrapped in a porous polytetrafluoroethylene membrane ("NTF-1122" manufactured by Nitto Denko) with a pore diameter of 0.2 μm cut to a size of 100 mm × 100 mm, and the opening of the wrapping was tied with a string. The weight of the adhesive sample (B) was calculated by subtracting the total weight (A) of the porous polytetrafluoroethylene membrane and the string measured in advance from the weight of this sample. The adhesive sample wrapped in the porous polytetrafluoroethylene membrane was immersed in about 50 mL of ethyl acetate at 23 ° C for 7 days, and the sol component of the adhesive was eluted outside the porous polytetrafluoroethylene membrane. After immersion, the adhesive wrapped in the porous polytetrafluoroethylene membrane was taken out, dried at 130 ° C for 2 hours, and allowed to cool for about 20 minutes, and then the dry weight (C) was measured. The gel fraction of the adhesive was calculated by the following formula. Gel fraction (%) = 100 × (CA) / B

[0150] <Storage modulus, loss tangent, and glass transition temperature> The adhesive sheets were laminated to a thickness of about 1.5 mm to prepare a measurement sample. Dynamic viscoelasticity measurements were performed under the following conditions using Rheometric Scientific's "Advanced Rheometric Expansion System (ARES)". From the measurement results, the storage modulus G' and loss tangent tanδ at each temperature were read. The temperature at which tanδ was maximized was determined as the glass transition temperature of the adhesive sheet.

[0151] (Measurement conditions) Deformation mode: Torsion Measurement frequency: 1Hz Heating rate: 5℃ / min Shape: Parallel plate 7.9mmφ

[0152] <Total light transmittance and haze> The adhesive sheet was attached to non-alkali glass (thickness 0.8-1.0 mm, total light transmittance 92%, haze 0.4%) to form a test piece, and the haze and total light transmittance were measured using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory). The haze of the adhesive sheet was determined by subtracting the haze of the non-alkali glass (0.4%) from the measured value. The measured value was used as the total light transmittance. The first adhesive sheet of each of the examples and comparative examples had a total light transmittance of 92%. The haze of the first adhesive sheet of Example 5 was 0.7%, and the haze of the first adhesive sheets of the other examples and comparative examples was 0.3%.

[0153] <Dielectric constant> The adhesive sheet was sandwiched between the copper foil and the electrode, and the dielectric constant was measured at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz under the following conditions in accordance with JIS K6911 using an Agilent Technologies Precision Impedance Analyzer 4294A. For the first adhesive sheets of Examples 1, 4, and 5 and Comparative Example 3, in addition to measurement at a temperature of 25°C, the dielectric constant was also measured at 20°C intervals in the temperature range of -40°C to 80°C. Electrode structure: 12.1mmΦ, 0.5mm thick aluminum plate Counter electrode: 3oz copper plate Measurement environment: Temperature 25℃, relative humidity 50%

[0154] <Adhesion to polyimide film> The release film on one side was peeled off from the adhesive sheet, a 25 μm thick PET film was attached, and the test piece was cut to a width of 10 mm and a length of 100 mm. The release film on the other side was peeled off from the test piece, and the adhesive sheet was pressed onto a transparent polyimide film (manufactured by Kolon Industries) with a thickness of 80 μm using a 2 kg roller. Using a tensile tester, the test piece was peeled off from the polyimide film at a tensile speed of 60 mm / min and a peel angle of 180° in an environment of 25°C, and the peel force was measured.

[0155] [Flexion retention test] A no-load U-shaped expansion and contraction tester for planar objects (manufactured by Yuasa System Equipment) was used to attach and fix bending jigs within a range of 20 mm from each end of the long side of the evaluation samples prepared in the examples and comparative examples (the central 60 mm area in the long side direction was left unfixed), and the samples were kept in a bent state with a bending radius of 1.3 mm and a bending angle of 180° so that the PET film side was on the inside, and a bending retention test was performed by keeping the samples in a constant temperature and humidity chamber at a temperature of 60°C and a relative humidity of 95% for 240 hours.

[0156] The samples after the bending retention test were visually inspected to confirm the presence or absence of peeling at the interface between the transparent polyimide film and the polarizing plate at the bent portion. In all samples where peeling was confirmed, peeling (gap) occurred from the end of the short side direction of the sample. For samples where peeling was confirmed, the length of the gap in the short side direction of the sample (mm) was measured. For samples where peeling was confirmed along the entire length of the short side of the sample, the length of the gap (gap distance) was 35 mm, and for samples where no peeling was confirmed, the gap distance was 0. For samples where peeling occurred from both ends in the short side direction, the gap distance was the longer length of the gap. In addition, in all samples, peeling was not confirmed at the interface where the PET film and the polarizing plate were attached.

[0157] [Evaluation Results] In the examples and comparative examples, the formulation of the adhesive composition used to prepare the first adhesive sheet is shown in Table 1, and the evaluation results of the first adhesive sheet and the evaluation results of the flexion retention test are shown in Table 2. Table 2 also shows the measurement results of the dielectric constant of the second adhesive sheet used to bond the polarizing plate and the PET film in the flexion retention test sample. Table 3 shows the measurement results of the dielectric constant of the first adhesive sheets of Examples 1, 4, 5 and Comparative Example 3 in the temperature range of -40°C to 80°C, as well as the ratio X of the minimum and maximum relative dielectric constant at each frequency, and the value of X at a frequency of 1 kHz (X 1kHz ) and the value of X at a frequency of 1 MHz (X 1MHz ) and X 1kHz / X 1MHz The figures show:

[0158] In Table 1, the components are described by the following abbreviations. LA: Lauryl acrylate 2HEA: 2-ethylhexyl acrylate BA: Butyl acrylate CHA: Cyclohexyl acrylate 4HBA: 4-hydroxybutyl acrylate 2HEA: 2-hydroxyethyl acrylate NVP: N-vinyl-2-pyrrolidone

[0159] [Table 1]

[0160] [Table 2]

[0161] [Table 3]

[0162] In all of Examples 1 to 5, the first adhesive sheet and the second adhesive sheet had a dielectric constant of 4.5 or less at a frequency of 10 MHz, and the gap distance was 2 mm or less. It can be seen from Table 3 that the first adhesive sheets of the examples had a small dielectric constant, and the temperature dependence and frequency dependence of the dielectric constant were also small. [Explanation of symbols]

[0163] 11,12,13,14 Adhesive sheet 31 Polarizing plate (circular polarizing plate) 51 Image display panel (organic EL panel) 54 Image display panel (touch panel integrated organic EL panel) 41 Touch Panel 71 Cover window 75 Case 101, 102 Image display device

Claims

1. A foldable image display device having a touch panel within a distance of 500 μm from a touch surface, a polarizing plate and a cover window are provided on a viewing side of the image display panel in this order from the image display panel side; a first adhesive sheet is provided on a viewing side surface of the polarizing plate, and a second adhesive sheet is provided on an image display panel side surface of the polarizing plate, each of the first adhesive sheet and the second adhesive sheet has a relative dielectric constant of 4.5 or less at a temperature of 25° C. and a frequency of 10 kHz; the first adhesive sheet is composed of an acrylic adhesive containing an acrylic base polymer, and the acrylic base polymer contains a hydroxy group-containing monomer in an amount of 10 parts by weight or less relative to a total of 100 parts by weight of monomer components; Image display device.

2. The image display device according to claim 1 , wherein the first adhesive sheet and the second adhesive sheet each have a ratio of a relative dielectric constant at 1 kHz to a relative dielectric constant at a frequency of 1 MHz at a temperature of 25° C. of 1.50 or less.

3. The image display device according to claim 1 or 2, wherein the first adhesive sheet and the second adhesive sheet each have a maximum relative dielectric constant in the temperature range of -40°C to 80°C at a frequency of 10 kHz that is 1.4 times or less than the minimum dielectric constant.

4. The image display device according to any one of claims 1 to 3, wherein the first adhesive sheet and the second adhesive sheet each have a ratio of maximum to minimum values ​​of relative dielectric constant in a temperature range of -40°C to 80°C at a frequency of 1 kHz that is 0.8 to 1.2 times the ratio of maximum to minimum values ​​of relative dielectric constant in a temperature range of -40°C to 80°C at a frequency of 1 MHz.

5. 5. The image display device according to claim 1, wherein the cover window has a thickness of 20 to 100 μm.

6. The thickness of the first adhesive sheet is greater than the thickness of the second adhesive sheet, The image display device according to any one of claims 1 to 5, wherein the first adhesive sheet has a thickness of 10 to 100 µm.

7. The first adhesive sheet is Storage modulus G' at 25°C and 1 Hz ２５ is 70 kPa or less, The glass transition temperature is −20° C. or lower. The image display device according to any one of claims 1 to 6.

8. The acrylic base polymer is a (meth)acrylic acid C based on 100 parts by weight of the total of the monomer components. １０－２０ 8. The image display device according to claim 1, comprising 5 to 55 parts by weight of a chain alkyl ester.

9. The acrylic base polymer is the (meth)acrylic acid C １０－２０ 9. The image display device according to claim 8, wherein the chain alkyl ester comprises lauryl acrylate.

10. The image display device according to any one of claims 1 to 9, wherein the acrylic base polymer contains 2 to 15 parts by weight of one or more polar group-containing monomers selected from the group consisting of hydroxy group-containing monomers, carboxy group-containing monomers, and nitrogen-containing monomers, relative to a total of 100 parts by weight of monomer components.

11. 11. The image display device according to claim 1, wherein the acrylic-based polymer has a crosslinked structure.

12. The image display device according to claim 11 , wherein the crosslinked structure is a crosslinked structure introduced by a polyfunctional (meth)acrylate.

13. 13. The image display device according to claim 1, wherein the acrylic pressure-sensitive adhesive further contains an acrylic oligomer having a glass transition temperature of 60° C. or higher.

14. 14. The image display device according to claim 13, wherein the content of the acrylic oligomer is 0.1 to 5 parts by weight based on 100 parts by weight of the acrylic base polymer.

15. 15. The image display device according to claim 1, further comprising a touch panel inside the image display panel.

16. 15. The image display device according to claim 1, further comprising a touch panel between the image display panel and the polarizing plate.

Images